He was injured in the experiment Gaster and Core Alphys was doing, a sharp knife accidentally cut my character/ core sans eye which made him lose sight in his left eye.....but when he came down with a very rare sickness he gains sight from his left eye again but he also gains a 2nd personality or as we call it the 2nd form of power.....when things get out of hand "he" would take over and fight whoever core sans is fighting